What’s the sense in giving a Demilich 80 HP making it vulnerable to Power word kill?

@IgorRuffinengo Hello: what’s the sense in giving a Demilich (CR 20 in lair) 80 HP making it vulnerable to one shot spells like Power word: kil? think of tomb of horrors – demilich isn't too tough if you know how to handle it and avoid traps that protect it